1. Field of the Invention
This invention relates to protecting data against copying and has particular application to protecting data transmitted through a network, such as hpermedia transmitted over a web-based network.
2. Related Art
It is known that data in the form of hypermedia such as hypertext, is often written in the hypertext language HTML and arranged in webpages that are provided by a server connected through a network to a client. The client may comprise a personal computer or other processing device capable of presenting the data retrieved from the server to a user. The network may comprise a local area network (LAN), a wide area network (WAN) or may comprise the Internet. For example, the World Wide Web comprises many servers connected over the Internet in a web, which have addresses in the form of universal resource locators (URL).
The hypertext information is arranged in webpages which include hotspots to allow the user to establish a link to another webpage, which may be located on the same or a different server, the routing to the document being achieved by use of a URL in the webpage at the hotspot.
Web clients typically access the hypermedia information using a browser. An overview of the World Wide Web and HTML is given in Chapter 1 of “HTML 3.2 and CGI Unleashed” J. December and M. Ginsberg 1996 (ISBN 1-57521-177-7).
As well known in the art, HTML webpages can display text, graphics and files of other descriptions such as video images, animated graphics and audio samples. Hypermedia have the significant advantage that the client can rapidly transfer viewing from one document to another by using a mouse to click on hotspots in the document, permitting the user to transfer from one web site to another, which may be at different physical locations.
The individual works which are displayed on the HTML pages may be copyright works. Because of the ease with which the copyright work may be viewed, transmitted and copied on the web, it is difficult for a copyright owner to enforce its copyright. For example, when a graphics file has been downloaded to a client, it may be readily copied onto the hard disc of a client's computer and replicated many times digitally, with no significant degradation from copy to copy.